In recent years, light-emitting devices have included quantum-dot emitting layers to form large area light emission. One of the predominant attributes of this technology is the ability to control the wavelength of emission, simply by controlling the size of the quantum dot. As such, this technology provides the opportunity to relatively easily design and synthesize the emissive layer in these devices to provide any desired dominant wavelength, as well as control the spectral breadth of emission peaks. This fact has been discussed in a paper by Bulovic and Bawendi, entitled “Quantum Dot Light Emitting Devices for Pixelated Full Color Displays” and published in the proceedings of the 2006 Society for Information Display Conference. As discussed in this paper, differently sized quantum dots may be formed and each differently-sized quantum dot will emit light at a different dominant wavelength. This ability to tune light emission provides opportunities for creating very colorful light sources that employ single color light emitters to create very narrow band and, therefore, highly saturated colors of light emission. This characteristic may be particularly desirable for visual displays, which typically employ a mosaic of three different colors of light-emitting elements to provide a full-color display.
Applications do exist, however, in which it is desirable to provide less saturated light emission and/or highly efficient light emission. One application for highly efficient, broadband light emission is general lighting devices. Within this application area, there are multiple requirements that such a light source must provide. First, the light source must provide at least one color of light that is perceived to be white. This white light requirement is typically specified in terms of color temperature or coordinates within either the 1931 Commission Internationale de l'Eclairage (CIE) chromaticity diagram or the 1976 CIE uniform chromaticity scale diagram. It is most desirable to create light sources that provide outputs having color coordinates that match typical blackbody radiators or typical daylight lighting conditions. The colors of light that exist during the day typically fall near a curve referred to as the Planckian Locus or black body curve within either the 1931 CIE chromaticity diagram or the 1976 CIE uniform chromaticity scale diagram. FIG. 1 shows the well-known CIE 1976 uniform chromaticity scale diagram illustrating the locus of monochromatic colors 10, and the Planckian Locus 12 for blackbody radiators having correlated color temperatures between 3000K and 10000K. Standardized lighting conditions that are desirable to attain, and that fall near this curve include D50, D65, and D93, where the numbers refer to the correlated color temperatures 5000K, 6500K and 9300K of the respective blackbody radiators. These points are shown in FIG. 1 as 14a, 14b and 14c, respectively. Secondly, the light source must be highly energy efficient. Within the industry, it is typical to employ light-emitting materials in a single package to form a single light source. For example, typical fluorescent light bulbs employ at least a red, green, and blue phosphor to form the desired color of light emission. Further, Organic Light Emitting Diode (OLED) light source prototypes have been demonstrated that employ multiple dopants in a single or in multiple layers to form a white light source. However, within these systems, the spectral characteristics of light emission are highly dependent upon the molecular structure of the material that forms the light-emissive layer or the dopant that is applied, and therefore device designers must select among relatively few materials, all of which have different radiant efficiencies and spectral emission characteristics. Finally, light sources capable of producing multiple colors of light are desired in many applications, including lamps for general purpose lighting that allow the user to easily and continuously adjust the color temperature of the light, and white light sources for displays wherein the white point of the display can be easily and continuously adjusted at the time of manufacture or during their use.
Light sources having color temperature regulation have been discussed by Okumura in US patent application 2004/0264193, entitled “Color Temperature-Regulable LED Light”. Within this disclosure, at least two different embodiments of different emitters are employed. In a first embodiment, a white LED, which is typically formed from a substance emitting blue or ultraviolet light together with a phosphorescent substance that absorbs this high energy light and re-emits lower energy broadband light, is employed together with typical narrow-band blue and a yellow LEDs. Within this embodiment, the emission from the phosphorescent substance forms a broadband emission necessary to have a reasonable color spectrum with respect to daylight. The light from the blue and yellow LEDs is then mixed with the white emitter to shift the color temperature of the light to a desired color temperature. In a second embodiment, three LEDs are employed, again with at least one of these having a phosphor coating to produce broadband light, one having a blue emission, a second having a yellow emission and a third having an orange emission. Each of these embodiments employs a light with at least three emissive LEDs, which are addressed independently from one another. Therefore, the light from which must properly balanced and mixed to produce the intended light output. The first embodiment in the Okumura disclosure provides three LEDs, the color points of which all are discussed as lying near a single line through a CIE chromaticity space. This fact reduces the tendency of the color of the light to shift away from the Planckian Locus if one LED fades faster than another since they lie along a line that is nearly parallel to the Planckian Locus. Unfortunately, this embodiment requires that either the blue or yellow LED be employed with the white LED. Therefore, if the system is not calibrated properly or if one of the LEDs ages at a different rate than another, it is likely that a luminance shift will occur at the point where one LED is turned off and the other is turned on. This has the potential to create a discontinuous change in color temperature as well as a sudden perceptual change in the perceived brightness of the lamp.
Duggal in U.S. Pat. No. 6,841,949, entitled “Color tunable organic electroluminescent light source” also provides for a color tunable light. This light, however, once again employs three colored emitters to provide the necessary color range. However, this embodiment employs a triplet of OLEDs with a diffusing layer to produce the range of colored light. Once again, the presence of three (e.g., red, green and blue) elements within the lamp to allow the lamp to obtain a range of CIE chromaticity coordinates, requires complex control of the current provided to the three independently-addressable, light-emitting elements. The proportion of light from the three light-emitting elements must be controlled to create the exact color coordinates of daylight sources, while factors such as unequal aging of the three lamps makes formation of daylight colors difficult.
In the open technical literature, studies have been published that demonstrate the ability to stack multiple layers of quantum dots within a single addressable light-emitting element, the individual layers being tuned to complementary wavelength bands to achieve the emission of white light. For example, in the article “From visible to white light emission by GaN quantum dots on Si(111) substrate” by B. Damilano et al. al. (Applied Physics Letters, vol. 75, p. 962, 1999), the ability to effect continuous tuning of a single light-emitting element during synthesis, from blue to orange, by control of the quantum dot size is demonstrated. A sample containing four stacked planes of differently sized quantum dots within a single light-emitting element was shown to produce white light, as demonstrated via photoluminescence spectra. Electroluminescent white light emission was not demonstrated, nor was continuous color tuning with a fixed material set.
US 2006/0043361 discloses a white light-emitting organic-inorganic hybrid electroluminescence device. The device comprises a hole-injecting electrode, a hole-transport layer, a semiconductor nanocrystal layer, an electron transport layer and an electron-injecting electrode, wherein the semiconductor nanocrystal layer is composed of at least one kind of semiconductor nanocrystals, and at least one of the aforementioned layers emits light to achieve white light emission. The semiconductor nanocrystal layer of this device may also be composed of at least two kinds of nanocrystals having at least one difference in size, composition, structure or shape. Organic materials are employed for the transport layers, whereas inorganic materials are employed for the nanocrystals and the electrodes.
U.S. Pat. No. 7,122,842 discloses a light emitting device that produces white light, wherein a series of rare-earth doped group IV semiconductor nanocrystals are either combined in a single layer or are stacked in individual RGB layers to produce white light. In one example, at least one layer of Group II or Group VI nanocrystals receives light emitted by the Group IV rare-earth doped nanocrystals acting as a pump source, the Group II or Group VI nanocrystals then fluorescing at a variety of wavelengths. Neither US 2006/0043361A1 nor U.S. Pat. No. 7,122,842 B2 demonstrates color tuning during device operation.
US 2005/0194608A1 discloses a broad-spectrum Al(1-x-y)InyGaxN white light emitting device which includes at least one broad-spectrum blue-complementary light quantum dot emitting layer and at least one blue light emitting layer. The blue-complementary quantum dot layer includes plural quantum dots, the dimensions and indium content of which are manipulated to result in an uneven distribution so as to increase the spectral width of the emission of the layer. The blue light emitting layer is disposed between two conductive cladding layers. Various examples are described in which the blue-complementary emission is achieved by means of up to nine broad spectrum emitting layers, and the blue emission is achieved by up to four blue emitting layers. Such a device does allow the possibility of color temperature variation during synthesis and manufacturing, however, it is achieved through a laborious selection of materials and these materials remain fixed after manufacture.
Therefore, there is a need for a simpler, efficient white light source of continuously adjustable color temperature.